As known in the art, syndiotactic polymers have a unique stereochemical structure in which monomeric units having enantiomorphic configuration of the asymmetrical carbon atoms follow each other alternately and regularly in the macromolecular main chain. Syndiotactic polypropylene was first disclosed by Natta et al. in U.S. Pat. No. 3,258,455. The Natta group obtained syndiotactic polypropylene by using a catalyst prepared from titanium trichloride and dietyl aluminium monochloride. A later patent to Natta et al., U.S. Pat. No. 3,305,538, discloses the use of vanadium triacetylacetonate or halogenated vanadium compounds in combination with organic aluminium compounds for producing syndiotactic polypropylene.
U.S. Pat. No. 3,364,190 to Emrick discloses a catalyst system composed of finely divided titanium or vanadium trichloride, aluminium chloride, a trialkyl aluminium and a phosphorus-containing Lewis base as producing syndiotactic polypropylene. U.S. Pat. No. 4,892,851 discloses a metallocene catalyst for producing highly crystalline syndiotactic polyolefins.
As disclosed in these patent references and as known in the art, the structure and properties of syndiotactic polypropylene differ significantly from those of isotactic polypropylene. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the sterochemical sequence of isotactic polypropylene is described as follows: 
Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature of an isotactic pentad is . . . mmmm . . . with each “m” representing a “meso” dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Using the Fischer projection formula, the structure of a syndiotactic polymer is designated as: 
In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each “r” represents a “racemic”, i.e., successive methyl groups on alternate sides of the plane.
The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene.
This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer that is soluble in xylene. Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms an essentially waxy product. While it is possible for a catalyst to produce all three types of polymer, it is desirable for a catalyst to produce predominantly syndiotactic or isotactic polymer with some atactic block fractions.
WO 98/02469 discloses use of metallocene catalysts in the formation of syndiotactic/atactic block homopolyolefins. The catalyst is a monocyclopentadienyl type catalyst. The cyclopentadienyl may be part of a fluorene ligand which is symmetrically substituted. The document is specifically concerned with 2,7 substituted fluorene ligands. However, the polymers produced using these catalysts do not have a melt temperature and are thus very sticky. This greatly reduces the utility of these polymers.
EP-A-0,931,814 discloses production of syndiotactic/atactic block polypropylene. The catalyst used in the production of the syndiotactic/atactic block polypropylene is a monocyclopentadienyl type catalyst. The cyclopentadienyl may be part of a fluorene ligand which is symmetrically substituted. The application is specifically concerned with 2,7 substituted fluorene ligands.
WO 96/00734 relates to group IV metal diene complexes and addition polymerisation catalysts produced therefrom. The document is particularly concerned with metallocenes having a cyclopentadienyl ligand and additionally a chelating alkenyl ligand.
Shiomura et al. disclose, in Macromol. Rapid Commun., 17, 9-14, (1996), inversion of stereoregularity in a metallocene catalyst. This document relates to an inversion of stereoselectivity of a particular metallocene from syndiospecific into isospecific by changing the cocatalyst. In particular, the document relates to an unsubstituted fluorene-containing metallocene.
EP-A-0,628,577 relates to fluorenyl-containing silyl bridged metallocenes, processes of using such metallocene to polymerise olefins and the syndiotactic polypropylene so produced. This document refers specifically to a 2,7-substituted fluorenyl type catalyst.
WO 93/19103 relates to a process and a metallocene catalyst system for the production of stereoregular polyolefins. The process permits improved control over tacticity, molecular weight and molecular weight distribution. In one example, the document relates to a polymerisation process employing a catalyst comprising an unsubstituted fluorene type ligand.